Zinc-copper alloy electroplating baths

ABSTRACT

The present invention relates to additives for electroplating baths used for the electrodeposition of zinc-copper alloys, and particularly to such alloys designed to be used as undercoats for subsequently applied metal coatings. The improvement comprises additives for incorporation into conventional zinc-copper electrolytes, comprising a mixture of a buffering agent selected from the group consisting of boric acid, alkali metal borates, alkali metal carbonates, alkali metal phosphates and glycine, a metal ion selected from the group consisting of nickel ion and cobalt ion, and ethylenediaminetetra acetic acid (as the alkali metal salt).

The present invention relates to the electrodeposition of zinc-copperalloys. More particularly, it relates to additives for use inelectroplating baths used for the electrodeposition of zinc-copperalloys.

Electrodeposited zinc-copper alloys are useful for numerous purposes,one particularly important use being as an undercoat on which othermetals are likewise applied by electrodeposition. When used for suchpurpose it is particularly important that the electrodeposited coatingof zinc-copper alloy be smooth, fine-grained, reflective and uniform inappearance as well in other physical properties even when applied over awide current density range. If otherwise, the metal coating applied overthe zinc-copper alloy will generally be defective and it may benecessary either to discard or else rework the coated article. This isparticularly the case where the zinc-copper alloy coating is coarsegrained, since the latter will generally show through the subsequentcoating. Many efforts have been made to overcome such defects ofprevious zinc-copper electroplating processes but without too muchsuccess.

It has now been discovered in accordance with the present invention thatby incorporating certain additives into the conventional zinc-copperelectroplating baths, comprising essentially zinc cyanide, coppercyanide, alkali metal cyanide and alkali metal hydroxide, thatzinc-copper alloys can be formed by electrodeposition having the abovedesired properties under operating conditions requiring less supervisionand with fewer rejects than has been the case with prior art methods.The additive composition which gives such improved results comprisesessentially a mixture of a buffering agent effective in theelectroplating bath within the 10 - 13 pH range and selected from thegroup consisting of boric acid, alkali metal borates, alkali metalphosphates, alkali metal carbonates and glycine; nickel or cobalt ion,preferably in the form of the metal complex; and a soluble salt ofethylenediaminetetra acetic acid, the latter appearing to serve both asa brightening agent and as a complexing agent for the nickel or cobalt.

The plating baths in which the above additive composition has been foundto be particularly effective have the following general composition:

    Zinc cyanide      13.5 - 40.4 gm/l                                            Copper cyanide    10.6 - 31.7 "                                               Sodium cyanide (free)                                                                           30.0 - 52.5 "                                               Sodium hydroxide  15.0 - 45.0 "                                           

The above bath gives a zinc-copper alloy upon electrodeposition havingthe composition: 70 - 30% zinc to 30 - 70% copper. A preferredcomposition for subsequent electrodeposition of metal has been found tobe of the order of 50 - 45% zinc to 50 - 55% copper.

The additive composition of the present invention used in the aboveplating bath has the following preferred composition:Boric acid, oralkali metal borate 0.010 - 40.0 gm/lNickel or cobalt metal (in form ofnickel or cobalt complex) 0.001 - 0.025 "

For the boric acid or alkali metal borate it has been found that thefollowing can be substituted:

    Alkali metal phosphate 0.1 - 20.0 gm/l                                        Glycine                1.0 - 15.0 "                                           Alkali metal carbonate 1.0 - 75.0 "                                       

It has further been found that the above additive may be improvedsomewhat by incorporating in the above additive composition 0.01 - 2.0gm/l of ethylenediaminetetra acetic acid (preferably in the form of itsalkali metal salt), which acts both as a brightening agent and as acomplexing agent for the nickel or cobalt.

The following specific examples are given to illustrate the effect ofthe individual ingredients of the additive composition of the presentinvention, as well as how the ingredients of the plating bath may bevaried and still obtain the improved results of the instant invention.It is understood also that the composition of the plating bath may bevaried in any conventional manner without departing from the scope ofthe present invention so long as the plating conditions are maintainedsubstantially as described herein.

In each of the first eleven examples the Hull Cell was used, employingan alloy anode containing 52% zinc and 48% copper, and mild airagitation of the anode during the plating operation. Zinc-coated 3 × 5inches steel plates were processed through the following cycle, theplating temperature being maintained at 72° - 80°F:

    1.        50% HCl strip - 1 minute                                            2.        Rinse                                                               3.        Electro-clean (reverse) - 1 minute                                  4.        Acid dip (10% sulfuric)                                             5.        Rinse                                                               6.        Copper strike                                                       7.        Rinse                                                               8.        Brass plate                                                         9.        Rinse                                                               10.       Hot water rinse                                                     11.       Dry                                                                 12.       Evaluate                                                        

EXAMPLE I

    Zinc cyanide         25.5    gm/l                                             Copper cyanide       22.5    "                                                Sodium cyanide       64.5    "                                                Sodium hydroxide     33.8    "                                                 Additive            None                                                 

The resulting deposit was dull to semi-bright at current densities of5 - 100 amperes/ft² and was regarded as unsatisfactory for subsequentplating.

EXAMPLE IIZinc cyanide 25.5 gm/lCopper cyanide 22.5 "Sodium cyanide 64.5"Sodium hydroxide 33.8 " Additive: Boric acid 15.0 "

The resulting deposit was semi-bright and reflective at currentdensities of 5 - 70 amperes/ft² and showed a decided improvement overthe use of no additive but still was not satisfactory for subsequentplating.

EXAMPLE IIIZinc cyanide 25.5 gm/lCopper cyanide 22.5 "Sodium cyanide64.5 "Sodium hydroxide 33.8 " Additive: Nickel ion (as complex) 0.005 "Ethylenediaminetetra acetic acid (as Na salt) 0.05 "

The resulting deposit was bright at current densities of 5 - 40amperes/ft² and dull over the remainder of the current density range butstill not satisfactory for subsequent plating.

EXAMPLE IV

    Zinc cyanide            25.5      gm/l                                        Copper cyanide          22.5      "                                           Sodium cyanide          64.5      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Boric acid             15.0      "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

The resulting deposit was smooth, bright and uniform and generallyexcellent over the current density of 5 - 80 amperes/ft² and made anexcellent undercoat for subsequent plating.

EXAMPLE V

    Zinc cyanide            25.5      gm/l                                        Copper cyanide          22.5      "                                           Sodium cyanide          64.5      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Sodium carbonate       30.0      "                                            Boric acid             15.0      "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

The resulting deposit was smooth, bright and uniform and generallyexcellent over the current density range of 5 - 100 amperes/ft².

The following examples show the results obtained with buffering agentsother than the boric acid used in the preceding examples.

EXAMPLE VI

           Zinc cyanide      25.5     gm/l                                               Copper cyanide    22.5     "                                                  Sodium cyanide    64.5     "                                                  Sodium hydroxide  33.8     "                                                   Additive:                                                                     Sodium phosphate 5.0      "                                       

The resulting deposit was semi-bright, smooth and reflective at currentdensities of 5 - 70 amperes/ft².

EXAMPLE VII

    Zinc cyanide            25.5      gm/l                                        Copper cyanide          22.5      "                                           Sodium cyanide          64.5      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Sodium phosphate       5.0       "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

The resulting deposit was bright at 5 - 50 amperes/ft² current density,semi-bright at 50 - 100 amperes/ft² and smooth and reflective throughoutthe current density range.

EXAMPLE VIII

           Zinc cyanide      25.5     gm/l                                               Copper cyanide    22.5     "                                                  Sodium cyanide    64.5     "                                                  Sodium hydroxide  33.8     "                                                   Additive:                                                                     Glycine          10.0     "                                       

The resulting deposit was dull at current densities of 5 - 10amperes/ft², semi-bright and reflective at 10 - 40 amperes/ft², and dullin the range 40 - 100 amperes/ft².

EXAMPLE IX

    Zinc cyanide             25.5     gm/l                                        Copper cyanide           22.5     "                                           Sodium cyanide           64.5     "                                           Sodium hydroxide         33.8     "                                            Additive:                                                                     Glycine                 10.0     "                                            Nickel ion (as complex) 0.005    "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                              0.05     "                                       

The resulting deposit was dull at current densities of 5 - 10amperes/ft², semi-bright, smooth and reflective at 10 - 60 amperes/ft²and dull at 60 - 100 amperes/ft².

EXAMPLE X

    Zinc cyanide            25.5      gm/l                                        Copper cyanide          22.5      "                                           Sodium cyanide          64.5      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Boric acid             15.0      "                                            Cobalt ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

The resulting deposit was bright, uniform and smooth at currentdensities of 5 - 80 amperes/ft².

EXAMPLE XI

           Zinc cyanide      25.5     gm/l                                               Copper cyanide    22.5     "                                                  Sodium cyanide    64.5     "                                                  Sodium hydroxide  33.8     "                                                   Additive:                                                                     Sodium carbonate 30.0     "                                       

The resulting deposit was bright to semi-bright at 5 - 30 amperes/ft²current density and dull throughout the remainder of the current densityrange. (Compare with Example V).

In the following examples the proportions of the constituents of theplating baths as well as those of the additive compositions were varied.In each instance excellent deposits of zinc-copper alloy were obtainedover a wide range of current density.

EXAMPLE XII

    Zinc cyanide            22.9      gm/l                                        Copper cyanide          29.6      "                                           Sodium cyanide (Free)   45.0      "                                           Sodium hydroxide        30.0      "                                            Additive:                                                                     Boric acid             0.15      "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

EXAMPLE XIII

    Zinc cyanide            22.9      gm/l                                        Copper cyanide          21.1      "                                           Sodium cyanide (Free)   45.0      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Boric acid             0.5       "                                            Nickel ion (as complex)                                                                              0.01      "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.05      "                                       

EXAMPLE XIV

    Zinc cyanide            22.9      gm/l                                        Copper cyanide          20.1      "                                           Sodium cyanide (Free)   41.3      "                                           Sodium hydroxide        28.1      "                                            Additive:                                                                     Boric acid             0.10      "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.075     "                                       

EXAMPLE XV

    Zinc cyanide            18.9      gm/l                                        Copper cyanide          19.0      "                                           Sodium cyanide (Free)   39.8      "                                           Sodium hydroxide        30.0      "                                            Additive:                                                                     Boric acid             0.2       "                                            Nickel ion (as complex)                                                                              0.002     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.1       "                                       

EXAMPLE XVI

    Zinc cyanide            33.7      gm/l                                        Copper cyanide          26.4      "                                           Sodium cyanide (Free)   58.8      "                                           Sodium hydroxide        33.8      "                                            Additive:                                                                     Boric acid             0.15      "                                            Nickel ion (as complex)                                                                              0.005     "                                            Ethylenediaminetetra                                                          acetic acid (as Na salt)                                                                             0.25      "                                       

It is understood, of course, that where the sodium compound is specifiedthat equivalent amounts of potassium metal compounds may besatisfactorily substituted.

The nickel is usually employed in the form of a nickel complex with theethylenediaminetetra acetic acid, a nickel salt being added to anaqueous solution of the latter, although other conventional nickelcomplexing agents may be satisfactorily used instead.

What is claimed is:
 1. An aqueous electroplating bath for the productionof zinc-copper alloys by electrodeposition which comprises 13.5 - 40.4gm/l zinc cyanide, 10.6 - 31.7 gm/l copper cyanide, 30.0 - 52.5 gm/lsodium cyanide (Free), 15 - 45 gm/l sodium hydroxide, 0.01 - 40.0 gm/lboric acid, 0.001 - 0.025 gm/l nickel ion and 0.01 - 2.0 gm/l ofethylenediaminetetra acetic acid (as alkali metal salt).
 2. The platingbath of claim 1 further including 1.0 to 75.0 gm/l of alkali metalcarbonate.
 3. An aqueous electroplating bath for the production ofzinc-copper alloys by electrodeposition which comprises 13.5 to 40.4gm/l zinc cyanide, 10.6 to 31.7 gm/l copper cyanide, 30.0 to 52.5 gm/lsodium cyanide (free) and 15 to 45 gm/l sodium hydroxide havingdissolved therein a mixture of additives comprising:a. an effectiveamount of at least one buffering agent selected from the groupconsisting of boric acid, alkali metal borate, alkali metal phosphate,alkali metal carbonate, glycine and mixtures thereof; b. about 0.001 toabout 0.025 gm./l of at least one metallic ionic material selected fromthe group consisting of nickel ion and cobalt ion and mixtures thereof;and c. about 0.01 to about 2.0 gm./l of ethylenediaminetetra acetic acid(as alkali metal salt).
 4. The aqueous electroplating bath of claim 3wherein said buffering agent is boric acid which is present in an amountranging from about 0.010 to about 40.0 gm/l.
 5. The aqueouselectroplating bath of claim 3 wherein said buffering agent is an alkalimetal borate which is present in an amount ranging from about 0.010 toabout 40.0 gm/l.
 6. The aqueous electroplating bath of claim 3 whereinsaid buffering agent is an alkali metal phosphate which is present in anamount ranging from about 0.1 to about 20.0 gm/l.
 7. The aqueouselectroplating bath of claim 3 wherein said buffering agent is glycinewhich is present in an amount ranging from about 1.0 to about 15.0 gm/l.8. The aqueous electroplating bath of claim 3 wherein said bufferingagent is an alkali metal carbonate which is present in an amount rangingfrom about 1.0 to about 75.0 gm/l.